Methylchlorosilane synthesis, also called direct synthesis, is carried out in fluidized bed reactors. During the process a part of fine particulate silicon and copper catalyst particles are together with metallic compounds, especially iron- and aluminium compounds, which are present in the silicon raw material, are removed from the reactor together with the gaseous reaction products, a mixture of silanes, and unreacted methylchloride. The solid materials are separated from the mixture of silanes and unreacted methylchloride in separating devices such as for example cyclones. In addition a residue will remain in the reactor, comprising silicon, copper and metal halides formed from compounds in the silicon raw materials and also comprising carbon deposits formed by decomposition of methylchloride. This residue is continuously or intermittently removed from the reactor.
Elemental copper, copper oxides, copper formate, copper hydroxides and other copper salts like copper chloride are used as a copper catalyst. The copper catalyst may further contain metals or metal compounds as activators, such as zinc, and zinc compounds, or promotors such as antimony, cadmium, phosphorus, tin, arsenic etc. in order to improve the reactivity and the selectivity of the produced silanes.
These residues have up till now normally been deposited in waste disposal sites. However, as the residues normally contain 1-10% by weight of copper, mainly in elemental form, copper may be leached from the residue which represents a danger for pollution of ground water. It is therefore no longer acceptable to deposit this type of residue in disposal sites.
A number of methods for recovering copper from the above mentioned residues have been proposed. Thus from German patent No. 901889 it is known to treat residue from the reactor in water and diluted hydrochloric acid under addition of chlorine gas in order to leach copper as divalent copper chloride and remove the remaining solid residue from the solution whereafter divalent copper chloride in the leach solution is reduced to cuprous chloride which is crystallized and used as a copper catalyst in the direct synthesis. The remaining solid residue, which mainly contains silicon, must, however, be deposited. In addition it is difficult to obtain a complete crystallization of cuprous chloride from the leach solution, making it necessary to subject the final solution to further treatment
From DE-A1 3523541 it is known a method for treatment of a hydrolysis residue from organochlorosilane production, where the residue is oxidized by sodium hypochlorite in order to leach copper from the residue. After removal of the solids from the leach solution, an alkaline earth- or alkaline hydroxide or an alkaline carbonate is added in order to precipitate copper oxides, hydroxides or carbonates. Also in this process the undissolved solid material, which mainly contains silicon, is deposited.
In U.S. Pat. No. 4,758,352 it is proposed to oxidize hydrolysis residue by using an oxygen containing gas. Also in this process only copper is recovered, while a silicon containing residue is deposited.
In DE-A 4205980 it is proposed to treat residue from direct synthesis by diluted sulphuric acid at elevated temperatures in order to dissolve copper and where copper can be precipitated as for example cuprous chloride or as copper-II-oxalate or where copper can be recovered by electrolysis. It is further disclosed in DE-A 4205980 that a solid silicon residue is obtained which can be used in metallurgical processes or which can be deposited. Chemical analysis of the obtained silicon residue is, however, not given.
By all the above-mentioned processes the residues are subjected to a leaching process in order to dissolve and recover copper while the undissolved solid matter is normally deposited. By the leaching process the leach solutions will, in addition to copper, contain a number of other metal ions such as iron and aluminium. The leach solutions must therefore be further purified before they can be discharged.